The proposed experiments will explore the phenomenon of accommodative micropsia, in which objects of some fixed angular size appear smaller when the eyes are accommodated and converged for near vision than when they are adjusted for distance vision. The objectives of the work are to determine the causes of accommodative micropsia, and to learn more about its time course and other properties. The first question to be answered is whether the phenomenon is directly related to accommodation, to convergence, or both: since these two processes normally occur together, special methods must be used if their separate contributions to the perceptual phenomenon under study are to be evaluated. In one experiment, accommodation will be stimulated under conditions where convergence would produce diplopia; it is well established that, under these conditions, a limited amount of accommodation can occur, unaccompanied by convergence. Analogous conditions can be set up for the production of convergence without accommodation (again within a limited range). Results of these experiments will show to which of the eye's muscular mechanisms the micropsia is related. Further experiments will show whether the phenomenon is brought about by means of changes in the retinal image during the operation of the eye's muscular mechanism (a possibility which will be evaluated by matching a real target to an afterimage), or whether a corollary discharge (that is, a neural signal related to a central command for accommodation or convergence) is the means by which the micropsia is brought about. The time course of accommodative micropsia will be studied in order to evaluate this last possiblity.